The present invention relates to methods for producing glass, especially float glass.
There are several different techniques for producing glass. In a typical furnace to produce glass, the glassmaking components are fed to a glassmaking furnace where they melt under various forms of heat to form molten glass. Gas bubbles formed during the melting step are removed during the fining step by growing the bubbles by gas generating reactions using fining agents such as sodium sulfate. Residual small gas bubbles are reabsorbed into glassmelt during the refining step as the glassmelt cools down and travels to a refiner through a submerged throat. For the product known by the term xe2x80x9cfloat glassxe2x80x9d, the glassmaking components are fed to a glassmaking furnace where they melt under various forms of heat to form molten glass. This molten glass exits the melting furnace through a narrow restriction, termed a xe2x80x9cwaistxe2x80x9d, then undergoes a temperature conditioning step prior to flowing out on top of a pool of molten metal, typically molten tin. The molten glass spreads out, xe2x80x9cfloatingxe2x80x9d on the molten metal, and forms a sheet or slab of glass. Sheets and slabs of glass are generally required to be of high quality, especially in being free of visible defects and unevenness in the top and bottom surfaces, and the float glass technique is generally considered to be an effective technique for producing sheets and slabs of glass which are of the necessary high quality.
Conventional glass furnaces have been fired by combusting fuel in air, wherein the oxygen required for the combustion is provided by the air. In recent years, many glass furnaces have successfully been operated by combusting the fuel with oxygen-enriched air (by which is meant oxidant comprising at least 80 vol. % oxygen) or with essentially pure oxygen. These furnaces, sometimes termed xe2x80x9coxy-fuelxe2x80x9d furnaces, provide numerous advantages including (but not limited to) greater efficiency, enhanced fining reactions, higher temperature, reduced volume of gas to be handled, and reduced formation of particulate matter and of NOx (by which is meant oxides of nitrogen such as NO, N2O, NO2, and the like)
The concentration of water vapor in the atmosphere of an oxy-fuel fired furnace is typically in a range between 50 to 65% as compared with 15 to 20% for an air-fuel fired furnace. A higher water vapor pressure in the atmosphere increases dissolution of water into glassmelt, which in turn enhances fining reactions to grow gas bubbles in the glassmelt. During the refining step, however, higher water content in glassmelt reduces the dissolution rate of water vapor in small residual gas bubbles back to glassmelt.In some instances small bubbles can find their way into the final product, resulting in higher than normal reject rates of the product.
A potential solution to this problem is lowering the partial pressure of water at the glass surface in the areas where the small bubbles need to be reabsorbed by blowing air into the furnace near the exit end (waist) to reduce the concentration of combustion products, and hence water, at the glass surface. The goal in this effort would be to minimize the amount of air required by finding the best location/technique for air injection.
Injecting air into the furnace may benefit the glass quality, but it brings along with it many other undesirable side effects. First and foremost is a reduction in energy efficiency. The added air will be heated in the furnace and leave at the furnace exhaust gas temperature, typically around 2700xc2x0 F. This puts an additional heat load on the furnace requiring more fuel to produce the same amount of glass. This effect is mitigated by preheating the air before injecting it into the furnace. However, that would require additional heat exchangers and other capital equipment to be implemented. In addition, the temperature to which the air could be preheated is limited by the heat exchanger design and will never attain the same level as the flue gas exit temperature.
The second effect that injecting air will have is to increase NOx emissions from the furnace. Part of the benefit of converting to oxy-fuel firing is to eliminate nitrogen from the combustion process and thereby allow reduced NOx emissions from the glass making process. Purposely introducing air into the process will diminish the value of using oxy-fuel firing.
The third effect of adding air to the process is that it will increase the flue gas volume leaving the furnace. This normally may not be a problem, unless there is some flue gas treatment equipment downstream of the furnace. Increasing flue gas volumes could require upgrading the flue gas handling equipment leading to additional cost of the manufacturing process.
Thus, there remains a need for a method of manufacturing float glass in an oxy-fuel fired furnace which avoids the possibility to form defects in the surface of the glass product.
One aspect of the present invention is a method of producing glass, comprising
providing a glassmelting furnace having a melting zone equipped with burners for combusting fuel and oxidant and an unfired refining zone,
feeding glassmaking materials into the melting zone,
feeding fuel and oxidant having an oxygen content greater than 80 vol. % oxygen to said burners and combusting said fuel and said oxidant in the melting zone to generate heat to melt the glassmaking materials in the melting zone to form molten glass, wherein the surface of the molten glass is exposed to the gaseous atmosphere in the melting zone and the water vapor content of the atmosphere at the surface of the molten glass is greater than 35 vol. %,
passing the molten glass into said refining zone,
cooling the molten glass in the refining zone without combusting fuel and oxidant in the refining zone, and
injecting oxidant having an oxygen content greater than 80 vol. % into the refining zone at a sufficiently low velocity below 50 feet per second to minimize mixing of said oxidant with gases above said oxidant to displace water vapor at the surface of the molten glass in the refining zone into the melting zone and lower the water vapor content of the atmosphere at the surface of the molten glass in the refining zone to less than 25 vol. %,
wherein the amount of oxidant injected into the refining zone and the amount of oxidant fed to said burners together equal from 85% to 120% of the stoichiometric amount needed for complete combustion of said fuel.
The invention involves using a portion of the oxygen necessary for combustion and injecting it into the refining end of the furnace to control the water vapor concentration in that end of the furnace. This diverted oxygen would then flow toward the burners and be consumed as part of the combustion process, thereby fully utilizing the gas required for blanketing the glass surface.
This method presents the following advantages, among others:
1) No additional fuel would be required as no extra gases are being added to the furnace.
2) Only minor modifications need be made to the oxygen piping system to implement the solution.
3) NOx levels should be reduced due to the staging effect produced by diverting some of the oxygen away from the high temperature burner area.
4) No increase in flue gas volume would occur.
5) Operating the burners fuel-rich tends to destabilize foam in glass furnaces. This may be an added benefit if the fuel-rich operation of the burners reduces the level of foam in the furnace and allows better heat transfer to the glass.
6) The added gas flow helps to cool the glass in the refining zone.